1. Field of the Invention
The present invention relates to microwave equipment and methods and more particularly to megawatt average power level microwave sources and waveguides that use windows to pass through microwave energy and that contain vacuums.
2. Description of Related Art
Gyrotrons and free-electron masers are electron-beam driven microwave sources for generating high intensity coherent electromagnetic radiation in the 30-300 gigahertz (Ghz) frequency range. Such microwave radiation assumes many of the characteristics of light, e.g., the ability to be focused, reflected, and diffracted. Such free-electron devices require microwave-transparent windows that can contain their vacuum envelopes against the atmosphere.
A broadband waveguide output window for a tunable low-power gyrotron in the millimeter and sub-millimeter wave region is described by J. Y. L. Ma, et al., in "Night moth eye window for millimetre and sub-millimetre wave region", Optica Acta, 1983 Vol. 130, pp. 1685-1695. Very thin mica windows in output waveguides can pass wide bandwidth energies, but the large cross section is very fragile. Unfortunately, these windows cannot handle high average power.
Nature has provided the night moth with a cornea in the eye that presents an array of cones that project from a surface. Such cones represent a way of achieving broadband anti-reflection, which is all good and well at optical frequencies. At microwave frequencies, certain problems in construction exist. J. Y. L. Ma, et al., proposed a gyrotron output window to separate vacuum from the outside world. They identified that such windows need to comprise a dielectric material that is mechanically strong and that has an appropriate absorption coefficient, e.g., fused quartz with a moth eye structure. Unfortunately, such structures cannot be used at high powers, because the cone tips overheat and cannot rid themselves of heat. The dielectric material itself is not an efficient heat conductor, and the only heat sink is at the edges of the window.
Grooved, or corrugated, surfaces for microwave generator windows were proposed by M. I. Petelin, et al., in "Surface corrugation for broadband matching of windows in powerful microwave generators", Int. J. Electronics, 1991, No. 5, pp. 871-873. Wave reflection in monochromatic sources is usually said to be eliminated by using a single edge-cooled disc that has a thickness of some multiple of the half-wavelength in the dielectric or by using a double disc with surface cooling where the gap between the discs forms a symmetric resonator at the monochromatic frequency. Broadband radiation is more a problem. Windows with surfaces shaped with periodic sequences of pyramids were dropped in favor of grooving the window surfaces, e.g., for simplicity in fabrication. Such grooving is shown for one surface only. The electric field of the incident polarized radiation is set perpendicular to the grooves for maximum effect. Experiments and computations conducted at Lawrence Livermore National Laboratory indicate that the relevant phenomenon do not manifest exactly as M. I. Petelin, et al., had theorized. It has been observed that incident radiation at microwave frequencies tends to diffract and bunch up into paths as it passes through the dielectric, e.g., with the highest energy banded into paths aligned with the crests of the grooves rather that the troughs. The grooved windows proposed by M. I. Petelin, et al., are not suitable for use at high powers, e.g., a megawatt; because the edge cooling or surface cooling cannot keep up with the heat dumped into the window.
The present inventors commented on the problem of barrier windows for vacuum envelopes in microwave tubes operated at high average power, especially at millimeter wavelengths. See, C. C. Shang and M. Caplan, "Electrical Analysis of Wideband and Distributed Windows Using Time-Dependent Field Codes", International Conference on Infrared and mm-waves, Essex, England, UK, Sept. 1993. Such windows experience severe thermal and mechanical stresses when operated at over the hundreds of kilowatts level. Grooved windows with grooved periods of .lambda./3 (at 140 Ghz) and groove depths of .lambda. were investigated. The corrugations were found to focus the radio frequency (RF) energy into the bulk dielectric portion of the window. See, FIG. 3 of the cite.
In U.S. Pat. No. 5,313,179, Charles Moeller describes a distributed microwave window that couples microwave power in the HE.sub.11 mode between two large diameter waveguides. The window provides a physical barrier between the two waveguides, e.g., without the need for any transitions to other shapes or diameters. The window comprises a stack of alternating dielectric and hollow metallic strips, brazed together to form a vacuum barrier. The metallic strips are tapered on both sides of the vacuum barrier to funnel the incident microwave power through the dielectric strips. The vacuum barrier is either normal to or tilted with respect to the waveguide axis. The wall of parallel strips are set perpendicular to the transverse electric field of the incident microwave power. A coolant is pumped through the metallic strips.
In a later U.S. Pat. No. 5,400,004, Charles Moeller further described corrugating the dielectric strips fronting the microwave power to reduce losses. FIGS. 7-9 of '004 show corrugations 60 comprised of ridges 62 that are very fine features that run parallel to the incident E-field. Such corrugations are said to function as alternating strips of air and dielectric and provide an effective dielectric constant that minimizes ohmic and dielectric losses associated with passage of microwave power through the window. The corrugated edges act as matching sections which reduce the internal standing wave and thus are said to reduce the internal stored energy and dissipation. Each of the metallic strips has a substantially hexagonal cross-sectional shape, with a first set of opposing sides sealed to respective sides of adjacent ones of the dielectric strips. A second and third set of opposing sides of the hexagonal-shaped metallic strips protrude into the interior of the waveguide with a taper.